Note (1.3, 1.4): plenty of code rely on this setting to indicate whether they should use the BagOStuff class (from $wgMemc) for caching, meaning they can not use Turck MMCache or eAccelerator, because this setting is only set if memcache is used, not if $wgMemc is valid. 1.5 may fix this.

Note (1.5): In all 1.5.x versions, this setting was not declared in DefaultSettings.php, but was still being used internally. This was rectified in 1.6.0.